A very important part of cathodic protection design is to check care- 

 fully for presence of other subsurface structures in the area that are not 

 to be protected or are owned by others. These are usually known as "foreign" 

 substructures and they may suffer from a side effect of cathodic protection 

 known as interference. The point to emphasize here is that a proposed 

 anode installation should not be placed closer than 90 meters from an 

 existing foreign substructure, either in soil or in water. This applies 

 particularly to a proposed impressed current anode installation. Also 

 check carefully for location of other cathodic protection systems in the 

 area, especially locations of anode beds. 



(2) Anode Choice . Whether to use galvanic anodes or impressed 

 current anodes to protect a given substructure normally will depend on 

 three factors, singly or in combination: 



(a) Amount of protective current required, 



(b) resistivity of the contacting electrolyte (soil or 

 water) ; and 



(c) electrical power availability. 



Current requirement can be calculated or, preferably, determined by 

 actual test. If calculated, the total exposed area in square meters must 

 be determined. If the substructure is bare steel in normal soil or normal 

 quiescent fresh or brackish water, a current density requirement of 10 

 milliamperes per square meter may be assumed. Total current required in 

 amperes would equal total exposed area in square meters multiplied by 0.01. 

 If all or part of the exposed steel is coated, an assumption of coating 

 quality must be made. For good quality, relatively undamaged coating, it 

 is customary to assume that the substructure is 1 percent bare for the 

 coated part and figure the total current requirement accordingly. Thus, a 

 coated substructure will require far less current for protection. The main 

 difficulty with calculating current requirements is that the 10 milliamperes 

 per square meter figure for bare steel may vary over a wide range, especially 

 in moving water where it is almost always higher, sometimes by orders of 

 magnitude. This is why an actual test for current requirement is preferable 

 to reliance on calculated values. 



Resistivity of the electrolyte (soil or water), in ohm- centimeters, 

 should be determined by measurement using the Wenner four-point method. 

 Resistivity is a measure of electrolyte resistance to current flow. The 

 higher the resistivity, the lower the output will be from a galvanic anode, 

 and the higher will be the voltage required to produce necessary current 

 from an impressed current anode. Reported resistivity of an area should be 

 an average of a number of measurements made at various locations in the 

 area. Measurements should be made by a person qualified by experience in 

 resistivity measurement for cathodic protection work. Table 56 shows 

 electrolyte resistivity and IR drop for bare pipe. 



Electrical power may be provided from several sources. Commercial 

 alternating current power is the most widely used and has the lowest cost. 

 Other sources are solar power (increasing in use as costs come down) , 

 thermoelectric, engine-driven generator, and windpower. Power availability 



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